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Gear selection issue Spitfire


Nick Jones
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Hi All,

Sons Mk IV Spitfire with 4 synchro, 3 rail box and D-type OD.  Box was rebuilt (by me) 3,500 miles ago and has been functioning pretty well.

He's now complaining of very stiff action in the 3rd/4th gate only once the car is fully warmed up.  It's fine when cold.  

It's stiff to get out of gear as well as in (esp 3rd) and the effect occurs moving or stationary, engine running or not.

So far we've removed the remote, which seems absolutely fine but has been cleaned and re-lubed anyway.  Testing with the tunnel cover off suggests a slight improvement but not cure (may just be cooler with cover off!).

Logic seems to suggest that the issue lies with the selector mechanism in the box-top though why it should be a problem now when previously working fine is a mystery.

Gearbox oil is at the correct level.

Any ideas?

Thanks

Nick

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there is a air bleed hole in the end of the top cover  in the selector shaft bores to allow air/oil out when the selector shaft moves in/out  if one is blocked then oil that gets in the cavity left by engaging 4th makes a hydraulic resistance to engaging 3rd and visa versa.

might this tie in with its ok when cold /unused , but develops when running

the other main culprit is clutch drag but you say its same engine on or off which excludes clutch clearing snags.

Pete

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Nick

3rd and 4th gate stiffens up slightly on my 3 rail box as it warms up. Nothing drastic but its there.

Pulled the remote off to check as you have done, but nothing obvious.

Will be interested in your further investigations/analysis/findings as always.

Ian F

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I have this problem, but only pulling the lever out of 3rd - going in is fine, as are all the other gears. The change is OK until the engine's hot, then shifting out of 3rd requires a sort of "push-pull" to release. It's like a brick wall - no slight spring (if you see what I mean) of the selector forks, and this made me think it was to do with the linking shaft. Perhaps a ridge has been worn in, although why that should happen in the steel of the shaft, not the alloy of the cover, I cannot fathom. Mine is a modified (shortened) remote, by the way.

Cheers, Richard

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in the ally top cover there is a a hole drilled for two ideas  one bleeds oil from being trapped and also lets you punch the welch plugs out  that cap the selector bores

there is between the 3 shafts a series of balls and pins to interlock the selection or you could get 2 gears at once 

if a pin or ball has been damaged then you can end up moving two forks at once ,  you have to strip the topp cover forks off and shafts out to check these

there is little in the remote if all the bushes and spherical are sound , ther are two  0 ring in the casting to seal the shaft ,they are fitted inside the rod bores into shallow machined grooves,, you have to undo the dowel bolt !!!  to strip these out ,

re fitting new ones the old shaft  normally chops the new ones and  this refurb is worse than left alone

Pete

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Pete

that's very interesting. I have had a similar problem with my new gearbox, difficult to engage third gear once its hot and has been run in fourth for a while.
Can you describe where the vent hole should be ? Is it close to the end of the ali housing where the 3 shaft are supported and does it vent to the outside of the gearbox?
Will pull the gearbox cover off and have a look around at the weekend.

cheers

mike

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its close to the end  , if you up end the top cover you wil see them 

this maybe a bum steer but many makes had hydraulic problems on sliding shafts so a drilled hole lets out trapped oil

But do make sure the sperical ball on the stick is lubricated /greased and the cup washers are not trying to shread it to bits the remote kits are pretty awful ,  having the lower pivot bolt in wrong way round will interfere with the remote casting and make the H gate more of a

]~)  gives 3rd 4th a curved engagement  

pete

 

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